Clematis addisonii Britton (Ranunculaceae) is a Virginia
endemic restricted to calcareous soils in a four county region
of the Ridge and Valley Province in Virginia. A two year
study of the reproductive biology of this species reveals that
it is self-compatible, showing no significant reduction in
fecundity following self-pollinations.

Morphological observations indicate that this species is
protogynous. In vivo pollen tube growth supports this
conclusion. Field observations suggest that the morphological
pistillate phase lasts significantly longer than the staminate
phase and is sufficient enough in length that cross-pollination
is likely to occur during the time period
preceding the staminate phase. The secretion of nectar from
the onset of anthesis enhances the probability that
outcrossing will occur prior to the presence of self-pollen in
flowers.

These findings suggest that, in spite of self-compatibility,
populations of Clematis addisonii are capable
of maintaining high levels of outcrossing by virtue of
protogyny and nectar secretion from the onset of anthesis.